The present invention relates to an apparatus for detecting an angular position of a crankshaft of an internal combustion engine by using an electromagnetic pick-up device disposed in opposition to :magnetic projections or teeth formed in and along an outer periphery of a rotatable disk. In particular, the invention is concerned with an improvement in the angular position detecting apparatus such that pulsating noise components superposed on the output signal of the electromagnetic pick-up device can be eliminated to thereby assure high reliability and accuracy in the detection of the angular position of the crankshaft and hence in the control of operation of the engine.
In general, an internal combustion engine such as a gasoline engine for a motor vehicle includes a plurality of cylinders in each of which an air fuel mixture is compressed and combusted at an optimal ignition timing. In this conjunction, there has already been proposed and widely used in practical applications a microprocessor-based engine control unit (also known as ECU in abbreviation) for the purpose of optimally controlling the ignition timing of igniters as well as the fuel injection sequence of fuel injectors for the individual engine cylinders.
Further, as a means for detecting the angular position of a crankshaft of the internal combustion engine (hereinafter also referred to simply as the engine) in order to obtain information about the operating positions of the individual cylinders, a variety of apparatuses have been proposed, among which there may be mentioned an angular position detecting apparatus composed of an electromagnetic pick-up device and a disk having an outer periphery formed with magnetic projections or teeth. In this type of angular position detecting apparatus, the electromagnetic pick-up device is usually disposed orthogonally in opposition to a plane of rotation of the disk because of limitation imposed on the space availability for the installation of the apparatus.
For a better understanding of the background of the invention, description will first be made of a known angular position detecting apparatus by reference to FIGS. 8 to 12, in which FIG. 8 is a schematic diagram showing the structure of a known angular position detecting apparatus, FIG. 9 is a plan view showing a geometrical configuration of a disk constituting a part of the apparatus, FIG. 10 is a schematic sectional view showing the structure of an electromagnetic pick-up device constituting another part of the apparatus, and FIGS. 11 and 12 are waveform diagrams for illustrating problems the known angular position detecting apparatus suffers.
Referring first to FIG. 8, a numeral 1 denotes a disk which rotates in synchronism with the crankshaft of an engine (not shown) and has a plurality of magnetic projections or teeth 2 formed in and along a circumferentially peripheral edge of the disk 1 at circumferentiatly equal intervals. In FIG. 9, only some of the teeth are shown. At this juncture, it should be noted that these teeth or projections may be replaced by recesses, dimples or the like while providing substantially the same results so far as they can bring about changes in the magnetic characteristic or the output of the electromagnetic pick-up device.
A reference numeral 10 denotes generally the electromagnetic pick-up device which is disposed orthogonally to the plane of rotation of the disk 1, facing in opposition to the magnetic teeth 2. As will be seen in FIG. 10, the electromagnetic pick-up device includes a core 11 having an outer end projecting toward the row of teeth 2 in a spaced and opposed relation therewith, a bobbin 12 disposed around the core 11, a coil 13 wound on the bobbin 12, a magnet 14 magnetically coupled to the core 11 at the inner end thereof, an electrode or terminal 15 connected to an end of the coil 13, a molded case 18 for fixedly packaging the above-mentioned components 11, 12, 13, 14 and 15 into an integrated structure, and a connector 17 for electrically connecting the electrode 15 to an engine control unit 30 (hereinafter referred to as ECU for short) (see FIG. 8).
In FIG. 8, a lead wire 19 extending from the ECU 30 is connected at one end thereof to the electrode 15 at the connector 17 for taking out an output signal A from the electromagnetic pick-up device 10. The ECU 30 includes a shaping circuit 20 to which the output signal A of the electromagnetic pick-up device 10 is input via the lead wire 19.
The shaping circuit 20 includes a comparator 21 for shaping the output signal A of the pick-up device 10 into a rectangular output signal B, a resistor 22 connected between the lead wire 19 and one input terminal of the comparator 21, and a capacitor 23 connected between the one input terminal of the comparator 21 and ground. The resistor 22 and the capacitor 23 cooperate to constitute a low-pass filter for eliminating external electromagnetic noise which is superposed on the output signal A of the pick-up device 10 on its way to the shaping circuit 20 via the lead wire 19 and which usually contains higher frequency components than that of the output signal A. The other input terminal of the comparator 21 is grounded so as to serve as a reference voltage input terminal.
The ECU 30 includes a processing unit (not shown) for arithmetically determining the angular position of the engine crankshaft on the basis of the rectangular waveform signal B output from the shaping circuit 20. Information about the angular position (crank angle) thus determined is utilized for the control of ignition timing for the engine cylinders and other purposes.
Next, referring to waveform diagrams illustrated in FIGS. 11 and 12, the operation of the known angular position detecting apparatus will be elucidated.
As the disk 1 rotates in synchronism with the crankshaft, the magnetic teeth 2 of the disk 1 successively pass by the outwardly projecting end of the core 11. As a result of this, a pulsating voltage is induced in the coil 13. The induced voltage is output as a signal A and supplied to the shaping circuit 20 of the ECU 30 via the electrode 15 and the lead wire 19.
In this conjunction, it is noted that the impedance of the shaping circuit 20 is selected to be on the order of 100 k.OMEGA. in order to ensure a sufficiently high voltage amplitude of the rectangular waveform signal while the impedance of the electromagnetic pick-up device 10 is usually in a range of 500 .OMEGA. to 1 k.OMEGA.. Consequently, external electromagnetic noise generated upon manipulation or operation of various switches and other electric parts is likely to be superposed on the output signal A on its way to the ECU 30 via the lead wire 19. Under the circumstances, the low-pass filter constituted by the resistor 22 and the capacitor 23 is so designed as to allow only the intrinsic output signal A to pass therethrough. The comparator 21 compares the output signal A having passed through the low-pass filter with the ground potential to thereby generate a rectangular waveform signal B containing a number of pulses corresponding to that of the magnetic teeth 2.
The output signal B from the comparator 21 should desirably exhibit an ideal rectangular waveform which corresponds to the magnetic tooth array 2, as is illustrated in FIG. 11. To this end, the disk 1 should have an ideal flatness and be free of mechanical vibration.
In that case, when the disk 1 is provided with, for example, 180 magnetic teeth 2, then one of the pulses of the rectangular waveform signal B represents an angle or angular increment of 2.degree.. Accordingly, the ECU 30 can discriminatively identify the angular position of the crankshaft (i.e. crank angle) and hence the stroke positions of the individual engine cylinders with a sufficiently high degree of accuracy, which in turn means that the cylinder operation can be optimally controlled with high reliability.
However, in practical applications, the disk 1 is usually deformed or warped more or less and additionally susceptible to vibrations of the engine. As a consequence, the output signal A will inevitably be superposed with a pulsating noise component having a frequency which is proportional to the engine rotation speed or the number of revolution per minute of the engine, as is indicated by a single-dot broken line curve in FIG. 12. Such a pulsating noise component has a frequency lower than that of the intrinsic output signal A. Obviously, the noise level increases as the engine rotational speed or the number of revolutions per minute becomes higher, as in the case of the intrinsic output signal A.
As described previously, the shaping circuit 20 is provided with the low-pass filter composed of the resistor 22 and the capacitor 23, as a result of which the level of the intrinsic output signal A tends to reach saturation as the frequency thereof increases. In contrast, the noise component whose frequency is inherently low can not reach the cut-off frequency of this low-pass filter. For this reason, as the engine rotation speed becomes higher, the level of the noise component increases correspondingly.
It will now be understood from the foregoing that the SN ratio of the output signal A becomes degraded more seriously as the engine rotation number increases, whereby the pulsating noise component tends to be emphasized or become remarkable to such an extent that the duty cycle of the pulse signal B output from the shaping circuit 20 becomes unstable or unsteady. In an extreme case, the pulses may disappear from the signal B, which of course presents a great obstacle to the engine control operation performed by the ECU 30.